Seawater pressure-driven desalinization apparatus and method with gravity-driven brine return

ABSTRACT

An apparatus and method of removing salt from seawater to produce potable freshwater. In the first preferred embodiment, a reverse osmosis system containing one or more reverse osmosis devices (“RODs”) is supported by a platform on the sea floor. In the second preferred embodiment, a cylinder is supported by a flotation device, and the reverse osmosis system is retained on the cylinder. In both the embodiments, an elongated brine return runs downhill on the sea floor. The RODs each contain a membrane that will allow water molecules, but not sodium and chlorine ions, to pass through. Due to the pressure differential, freshwater passes through the membranes by reverse osmosis, and is pumped out of the pressure hulls to a storage facility onshore.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a Divisional Application of Utility patentapplication Ser. No. 09/716,230, filed Nov. 21, 2000, which was aContinuation-In-Part of Utility patent application Ser. No. 09/287,658,filed Apr. 7, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a reverse osmosis method ofremoving the salt from water in the ocean or inland bodies of saltwater, using the pressure of the seawater itself, and the force ofgravity.

[0004] 2. Description of the Prior Art

[0005] Due to the shortage of freshwater in the southwestern UnitedStates and other arid parts of the world, there have been numerousinventions for desalinating sea water, by reverse osmosis, distillation,and other means. However, desalinization remains an expensive process.The concentrated brine produced as a by-product of desalinization canitself contribute to pollution of the environment in onshore facilities.The production of electricity or other forms of energy consumed indesalinization can also contribute to pollution of the air, water andland.

[0006] U.S. Pat. No. 3,171,808, issued on Mar. 2, 1965, to Henry W.Todd, discloses an apparatus for extracting fresh water from ocean saltwater, using vanes that are not included in the present invention.

[0007] U.S. Pat. No. 3,456,802, issued on Jul. 22, 1969, to Marc Cole,discloses an apparatus for desalinization by submerged reverse osmosis,without the gravity-driven brine return of the present invention.

[0008] U.S. Pat. No. 4,125,463, issued on Nov. 14, 1978, to James W.Chenoweth, discloses a reverse osmosis desalinization apparatus andmethod, that is placed in a well hole for desalinating salty groundwater.

[0009] U.S. Pat. No. 4,335,576, issued on Jun. 22, 1982, to Harold H.Hopfe, discloses a device for producing freshwater from seawater whichfloats on the surface of the sea. It derives the energy fordesalinization from the motion of the waves on the surface of the water.Movement of the water on the surface causes reaction plates to move, andthe movement is ultimately transmitted to pistons that move in cylindersto exert pressure on seawater to force reverse osmosis.

[0010] U.S. Pat. No. 4,452,969, issued on Jun. 5, 1984, to FernandLopez, discloses a reverse osmosis apparatus for producing freshwaterfrom seawater, which is designed to be temporarily submerged, as on afishing line. U.S. Pat. No. 4,770,775, issued on Sep. 13, 1988, toFernand Lopez, discloses another apparatus for the production offreshwater from seawater, which is also designed to be temporarilysubmerged, and has a chamber that expands as freshwater is produced.Both of these apparatuses use the pressure of the seawater itself toforce reverse osmosis.

[0011] U.S. Pat. No. 5,167,786, issued on Dec. 1, 1992, to William J.Eberle, discloses a wave power collection apparatus, which is anchoredin the sea floor, and in one embodiment desalinates seawater by reverseosmosis. The movement of floats is used in that embodiment to turn agenerator which produces electricity to power pumps that force seawaterthrough a membrane in a reverse osmosis unit.

[0012] U.S. Pat. No. 5,229,005, issued on Jul. 20, 1993, to Yu-Si Fokand Sushil K. Gupta, discloses a process for the desalinization ofseawater, by lowering reverse osmosis devices into the ocean by means oflines attached to pulleys, and raising them again by the same means toremove the freshwater produced. The pressure of the seawater itself isused to force reverse osmosis of the seawater across a membrane toproduce freshwater.

[0013] U.S. Pat. No. 5,366,635, issued on Nov. 22, 1994, to Larry O.Watkins, discloses a desalinization apparatus and means in which aseparator is placed on the sea floor, and the pressure at the sea flooris used to force seawater through a membrane to form freshwater byreverse osmosis, which is then pumped out.

[0014] U.S. Pat. No. 5,914,041, issued on Jun. 22, 1999, to Dennis H.Chancellor, discloses channel based reverse osmosis, in which reverseosmosis units are placed within a deep channel. The channel containsunpurified liquid (such as salt water) at a level such that the pressureacross the membranes of the reverse osmosis units causes purified liquid(such as fresh water) to accumulate in cavities in the reverse osmosisunits, from which it is emptied and pumped to the surface.

[0015] U.S. Pat. No. 5,916,441, issued on Jun. 29, 1999, to Roger J.Raether, discloses an apparatus for desalinating salt water in a mineshaft.

[0016] U.S. Pat. No. 5,944,999, issued on Aug. 31, 1999, to Dennis H.Chancellor, Marc Chancellor and Jacquetta M. Vogel, discloses a modularfiltration system, in which the weight of the fluid being filtered isused to drive the filtration process.

[0017] British Patent No. 2,068,774, published on Aug. 19, 1981, to JoseLuis Ramo Mesple, discloses an apparatus for desalinating water byreverse osmosis in cells located deep underground, utilizing thepressure resulting from the water being deep underground.

[0018] The Osmotic Pump, by Octave Levenspiel and Noel de Nevers,Science, January 1974, Volume 183, Number 4121, pages 157-160, disclosesthe idea of using the weight of sea water to drive a desalinizationprocess, but does not disclose the structures and mechanisms of thepresent invention.

[0019] The present invention is distinguishable from the prior artcited, in that only it takes advantage of the fact that the concentratedbrine produced as a by-product of reverse osmosis desalinization isheavier than seawater to reduce the energy consumed in desalinization,by means of a gravity-driven brine return. None of the above inventionsand patents, taken either singly or in combination, will be seen todescribe the present invention as claimed.

SUMMARY OF THE INVENTION

[0020] The present invention is an apparatus and method of removing saltfrom seawater to produce potable freshwater. In the first preferredembodiment, a reverse osmosis system containing one or more reverseosmosis devices (“RODs”) is supported by a platform on the sea floor. Inthe second preferred embodiment, the cylinder is supported by aflotation device, and the reverse osmosis system is retained on acylinder. In both the embodiments, an elongated brine return runsdownhill on the sea floor. The RODs each contain a membrane that willallow water molecules, but not sodium and chlorine ions, to passthrough. Check valves allow sea water to pass from outside the reverseosmosis system into the RODs. Due to the pressure differential, watermolecules pass through the membranes by reverse osmosis, while salt isleft behind, and freshwater is pumped out of the pressure hulls to astorage facility on shore (or where ever else it is needed).

[0021] Accordingly, it is a principal object of the invention to providea means for reducing the energy required to desalinate seawater.Conventional desalinization plants, located on or near the seashore,require four pumping processes: first, pumping the seawater to theplant; second, pumping to raise the pressure high enough for the RODs tooperate; third, pumping the brine back out to sea; and fourth, pumpingthe freshwater to a reservoir or a treatment facility for furtherpurification, and ultimately to the consumer. The present inventioneliminates all but the fourth pumping process. While prior inventions ofoffshore desalinization apparatus, as in U.S. Pat. No. 5,366,635 toWatkins, will also eliminate the first and second processes, only theinstant invention will also eliminate the third process of pumping outthe brine, without requiring that energy be expended in raising theRODs, as in U.S. Pat. No. 4,452,969 to Lopez and U.S. Pat. No. 5,229,005to Fok et al.

[0022] It is second object of the invention to provide a means forreducing the need for using expensive real estate on or near theoceanfront for desalinization facilities. As no oceanfront ornear-oceanfront property is used exclusively for the process, most realestate costs associated with desalinization plants can be avoided. Someoffshore site leasing may be required, but this cost should be muchlower than for offshore sites involved in petroleum or mineralextraction.

[0023] It is a third object of the invention to provide a means formaking the expansion of desalinization facilities easier and lessexpensive. As each platform must have a clear navigation zone around it(as most jurisdictions require by law), sufficient space for attachingadditional pressure hulls to the cylinder will be available and facilityexpansion considerably eased. The expansion of a facility is limitedonly by the number of pressure hulls that can be fitted onto thecylinder at appropriate depths, rather than allowances made by a zoningcommission with many other constituents to satisfy, as may the case witha land-based desalinization facility.

[0024] It is a fourth object of the invention is to provide a means forreducing the cost of desalinizing seawater by centralizing maintenancefacilities, as the pressure hulls can be removed and taken to a centralfacility for maintenance, rather than the on-site maintenance requiredby conventional shore-based desalinization plants.

[0025] It is a fifth object of the invention to reduce pollution of theshoreline from the release of concentrated brine by desalinizationplants. Conventional onshore desalinization facilities pump their brineout to sea through a bottom-laid pipeline, which releases the brine onor near the ocean floor. Releasing the brine near the ocean floorincreases the area affected by the brine's toxicity. Existing methods toreduce the toxic effects add to the cost of desalinization throughgreater plant infrastructure requirements or reduced process efficiency.The present invention allows an offshore desalinization facility torelease its brine into mid-water, where mixing with the ocean current ismore efficient, with fewer effects upon bottom-dwelling flora and fauna.Because the facility can be located offshore, ocean currents and tidalaction will thoroughly mix the brine back into the surrounding seawater,and the overall impact of increased salinity from the brine releasecould be infinitesimal as little as two or three kilometersdown-current.

[0026] It is an object of the invention to provide improved elements andarrangements thereof in an apparatus for the purposes described which iscost effective, dependable and fully effective in accomplishing itsintended purposes.

[0027] These and other objects of the present invention will becomereadily apparent upon further review of the following specification anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic environmental front elevational view of thefirst preferred embodiment of the invention.

[0029]FIG. 2 is a schematic environmental front elevational view of thesecond preferred embodiment of the invention.

[0030] Similar reference characters denote corresponding featuresconsistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention is an apparatus and method of removing saltfrom seawater to produce potable fresh water. It may be used in eitherthe oceans or in inland bodies of salt water.

[0032]FIG. 1 is a schematic environmental front elevational view of thefirst preferred embodiment of the invention, in which a single reverseosmosis system 10 is elevated above the sea floor A by a platform 12.The reverse osmosis system has an external skin and contains on or morereverse osmosis devices (“RODs”) (not shown in the drawings). The RODseach having a selectively permeable membrane surrounding a brineenclosure. The membrane allows water molecules, but not sodium andchlorine ions, to pass through. (Other substances may also be filteredout of the seawater, depending on the characteristics of the membrane.)A seawater inlet (not shown in the drawings) allows water to pass intothe reverse osmosis system after passing through a screen for fish andlarge matter, a pre-filter for silt and particulate matter, amicro-filter for bacteria and suspended solids, and a check valve.Desalinated water is removed from the fresh water enclosures of the RODsthrough a fresh water return 14 by at least one fresh water pump (notshown in the drawings) which may be located in the reverse osmosissystem, along the fresh water return, or on shore. Water having anincreased concentration of salt is removed from the brine enclosures ofthe RODs through a brine return 16. As shown in FIG. 1, the brine returnpasses down the platform, over the sea floor, and has an outlet 18 wherethe brine is released into the surrounding seawater, preferably at aconsiderable distance (as indicated by the pair of jagged lines) fromthe reverse osmosis system (so that the brine does not soon become mixedback in with the seawater being desalinated) and over an undersea cliffB or other area of the sea floor having a lower elevation the sea flooron which the platform rests. The reverse osmosis system is located farenough below the surface of the sea C that the weight of the overlyingsea water creates sufficient pressure for reverse osmosis to occuracross the membranes in the RODs. A pressure differential is maintainedacross the membranes by the fresh water pumps. Initially, the brine ispumped out of the brine enclosures by a brine pump (not shown in thedrawings) until the brine return is filled (or “charged”) with brine.Then the brine pump can be turned off, while the inlet 20 of the brinereturn remains open, and the brine will continue to flow downhillthrough the brine return under the force of gravity, because it isheavier than the surrounding seawater. This saves the energy that wouldotherwise be needed to pump out the brine. (Note that the weight of thebrine in the brine return, and the drop in depth from the brine return'sinlet to its outlet, must be great enough that the downward pressure ofthe brine under the force of gravity exceeds the back pressure acrossthe membranes in the RODs that is maintained by the fresh water pump.)

[0033]FIG. 2 is a schematic environmental front elevational view of thesecond preferred embodiment of the invention, which is similar to thefirst preferred embodiment, except that rather than being supported by aplatform on the floor, it is supported by a flotation device 22 on thesurface of the sea. A channel 24, which is preferably a hollow verticalcylinder with an open top 26 and a closed bottom 28, is retained on theflotation device, and the reverse osmosis system 10 is retained on thechannel below the flotation device. The top of the channel passes abovethe surface of the sea. The reverse osmosis system is held in place bylines 30 connected to anchors 32 in the sea floor. Rather than beingpumped directly into the brine return, brine is pumped into the channelthrough an outlet (not shown in the drawings) of the brine enclosures.The inlet of the brine return is connected to the channel, so that brinecan then flow from the channel into the brine return. Once both thechannel and the brine return are filled with brine, the brine pump canbe turned off, while the outlet of the brine enclosures remains open,and brine will continue to flow out through the brine return, as in thefirst preferred embodiment. Because the brine is denser than thesurrounding seawater, the surface of the brine in the channel will bebelow the surface of the sea, and it will have an increased pressuregradient (i.e., the pressure of the brine will increase more rapidlyover a shorter vertical distance than the pressure of the seawater).This has the consequence that the channel can (and should) be positionedin relation to the reverse osmosis systems in such a manner that thepressure in the channel is less than the pressure in the brineenclosures in the RODs, thus causing the brine to flow out from the RODsinto the channel, and from the channel out through the brine return.

[0034] The earth's gravity will cause the brine in the channel to flowout of the bottom opening until the weight of the brine in the channelequals the weight of an equivalent column of water in the sea outsidethe channel. As brine continually flows into the channel when theinvention is in operation, the weight of the brine in the channel willcontinue to be heavier than that an equivalent column of seawateroutside, and brine will continue to flow out. If there were no currentsin the sea, the salinity of the sea in the immediate area around thechannel could eventually rise to almost the degree of salinity in thechannel (though not to complete equality, due to diffusion of saltthrough the seawater). This would cause the level of brine in thechannel to rise to almost the level of the sea outside the channel, andit would be necessary to reactivate the brine pumps for desalinizationto continue. (This might actually happen in inland bodies of salt water,which lack drainage to the oceans, if desalinization were carried out ona massive scale over a long period of time.) Thus, the present inventionderives its energy savings, not out of nothing, as would a perpetualmotion machine, but from the force of the earth's gravity, from oceancurrents land interlayer mixing that are driven by electromagneticradiation produced by nuclear reactions in the sun, and from diffusionmade possible by random movements of molecules and ions in the seawaterthat are also driven by heat from the sun.

[0035] It is to be understood that the present invention is not limitedto the embodiments described above, but encompasses any and allembodiments within the scope of the following claims.

MATHEMATICAL APPENDIX The Power Advantage Of an “At-Depth”Desalinization Plant Versus A Shore-Based Plant

[0036] Assumptions are as follows:

[0037] 1. Both plants produce the same quantities of desalinated water.

[0038] 2. The product water arrives at shore at the same pressure.

[0039] 3. There is a method to return the brine (effluent) to the ocean.

[0040] 4. The internal process is the same, the only difference in thefacilities is the location.

[0041] 5. The process requires a delta pressure of 850 psi across theelement.

[0042] 6. The pressure loss through the tube side, brine side(effluent), is 35 psi.

[0043] 7. The process produces about a 30% yield of product, i.e.De-salinated water.

[0044] 8. For simplicity the production will be based on 1 m³ persecond.

[0045] Calculations basic to both processes:

[0046] Product=1 m³/s therefore the initial supply of seawater=1 m³/s/.3=3.33 m³/s

[0047] And by difference the effluent of brine=2.33m³/s

[0048] As an approximation, 1 m³/s water requires about 6.9 kW to raisethe pressure by 1 psi.

[0049] I. Analysis of a typical shore based facility is as follows. Itwill be assumed that the shore-based facility can recapture enoughexcess energy from the effluent to do all auxiliary pumping and toovercome frictional losses. Also to be conservative it is assumed thatfrictional forces of the supply is so small as to be negligible.Therefore the entire energy cost will be assumed to be in pressurizingthe supply, as follows:

3.33 m³/s×850 psi×6.9 kW/(psi×m³/s)=19.5 MW

[0050] II. Analysis of the “At-depth” plant will include a discussion ofall pressure losses, since the process is streamlined and there is noexcess energy from any part of the process. Only the desalinated(product) water need be pumped (pressurized) to the 850 psi level. Thethree parts to this equation are the effluent “make-up pressure”, theproduct pumping, and the frictional losses due to the piping of theproduct to shore. The pipe can be sized to maintain a flow rate of about1 mls.

[0051] a. Product power requirement=850 psi×1 m³/s×6.9 kW/(psi×m³/s)=5.9MW

[0052] b. Effluent power requirement=35 psi×2.33 m³/s×6.9kW/(psi×m³/s)=0.5 MW

[0053] c. Power required to overcome frictional losses:

[0054] friction loss=fluid density×coefficient offriction×length/diameter×velocity squared/2

[0055] Re=fluid velocity×pipe diamet

[0056] Re=1 m/s×0.56 m×1000 kg/m3 /1000×10⁻⁶ Pa s=5.6×10⁵

[0057] Relative Roughness=mean roughness/pipe diameter=0.04 mm/0.56m=7.1×10⁻⁵

[0058] And therefore from standard tables the Coefficient of friction is0.014

[0059] Therefore the frictional loss is:

1000 kg/m³×0.014×11000 m/0.56 m×(1 m/s)²/2=138,000 m/kg s²−138000 Pa

138000 Pa×1 psi/6895 Pa=20 psi

[0060] And Frictional losses=20 psi×1 m³/s×6.9 kW/(psi×m³/s)=0.1 MW

[0061] Therefore the total power requirement is:

5.9 MW+0.5 MW+0.1 MW=6.5 MW or ⅓ of the shore system's 19.5 MW

[0062] Discussion:

[0063] 1. Pumping efficiency was ignored for simplicity since the sameinefficiencies would govern both systems equally and not change thestatistical results.

[0064] 2. For both systems the product required the entire 850 psi,therefore both systems have lost the 5.9 MW in forcing the separationprocess and it is non-recoverable.

[0065] 3. The additional power required by the “at-depth” system is inadding “make-up pressure” to the effluent and overcoming frictionallosses, together about 10% of the total power requirement.

[0066] 4. The shore-based system pressurizes the entire supply to the850 psi. More than 3 times the mass required by the “at-depth” system.

[0067] 5. The “at-depth” system releases the effluent immediately at theend of the separation process, while the shore based system must returnthe effluent to a safe mixing zone with enough energy to ensure propermixing, which requires more power.

[0068] 6. If the shore based system were able to recycle as much as 50%of the “lost” power back into the system, then the “at-depth” systemwould still require only ⅔s as much power.

I claim:
 1. An apparatus for desalinating seawater, comprising: at leastone seawater inlet, with at least one check valve; at least one membranethrough which water molecules can flow, but through which sodium andchlorine ions cannot flow; at least one fresh water enclosure, withinwhich water that has been desalinated by passing through the membrane,can be collected and separated from salt water; at least one freshwaterpump for removing desalinated water from the freshwater enclosure; atleast one brine enclosure, within which water that has not passedthrough the membrane, and has an increased concentration of salt, can becollected and separated from water with a lower concentration of salt;at least one brine return having an inlet connected to the brineenclosure and an outlet lower in elevation than the brine enclosure; andat least one brine pump for removing water having an increasedconcentration of salt from the brine enclosure to the brine return. 2.An apparatus for desalinating seawater according to claim 1, wherein thebrine pump can be turned on and turned off, and the inlet to the brinereturn is open when the brine pump is turned off, whereby water havingan increased concentration of salt can continue to flow out through thebrine return after the brine pump is turned off.
 3. An apparatus fordesalinating seawater according to claim 2, wherein the brine return isan elongated channel that passes along the sea floor, from an area wherethe sea floor has a higher elevation near the inlet of the brine return,to an area where the sea floor has a lower elevation near the outlet ofthe brine return, whereby the force of gravity will cause water havingan increased concentration of salt to continue to flow through the brinereturn after the brine pump is turned off.
 4. An apparatus fordesalinating seawater according to claim 3, wherein the membrane, thefresh water enclosure, and the brine enclosure, are contained within areverse osmosis system, and the reverse osmosis system is retained on aplatform resting on the sea floor.
 5. An apparatus for desalinatingseawater according to claim 4, wherein the inlet has a screen and atleast one filter through which seawater must pass before it can reachthe membrane.
 6. An apparatus for desalinating seawater according toclaim 5, wherein the membrane, the fresh water enclosure, and the brineenclosure, are contained within a reverse osmosis system, and thereverse osmosis system is supported by a floatation device.
 7. Anapparatus for desalinating seawater according to claim 3, wherein thereis a channel retained on the flotation device, and the reverse osmosissystem is retained on the channel below the flotation device.
 8. Anapparatus for desalinating seawater according to claim 7, wherein thechannel has at least one top opening, the brine enclosure has an outletconnected to the channel through which water having an increasedconcentration of salt can pass from the brine enclosure into thechannel, and the inlet of the brine return is connected to the channel.9. An apparatus for desalinating seawater according to claim 8, whereinthe channel has no openings other than the top opening, an opening forthe outlet of the brine enclosure, and an opening for the inlet of thebrine return.
 10. An apparatus for desalinating seawater according toclaim 9, wherein the inlet has a screen and at least one filter throughwhich seawater must pass before it can reach the membrane.
 11. A methodof desalinating seawater, comprising the steps of: allowing seawater topass through an inlet with a check valve into a brine enclosure; reverseosmosis of the seawater as water molecules pass from the brine enclosurethrough a membrane into a fresh water enclosure, while the membraneblocks the passage of sodium and chlorine ions; maintaining a pressuredifferential across the membrane by pumping desalinated water out of thefresh water enclosure; and pumping water having an increasedconcentration of salt out of the brine enclosure to a brine return thatis an elongated channel that passes along the sea floor, from an areawhere the sea floor has a higher elevation near an inlet of the brinereturn, to an area where the sea floor has a lower elevation near anoutlet of the brine return; whereby, if the pumping is discontinuedafter the brine return is filled with the water having an increasedconcentration of salt, the force of gravity will cause the water havingan increased concentration of salt to continue to flow through the brinereturn.
 12. A method of desalinating seawater according to claim 11,wherein the brine enclosure is directly connected to the inlet of thebrine return.